1. Field of the Invention
The present invention relates to a data backup device, a data backup method, and a recording medium storing data backup program. In particular, the present invention relates to backing up data between plural data centers in a disaster recovery system.
2. Description of the Related Art
A disaster recovery system with a primary data center and backup center (secondary data center) is known. The secondary data center backs up the primary data center in case of a disaster such as earthquake or flood.
In many disaster recovery systems having the secondary data center, the primary data center and the secondary data center are geographically separate from each other. And replicated data of a memory unit in a storage device of the primary data center is stored in the memory unit in the storage device of the secondary data center.
The following two types of methods are used for copying data from the memory unit in the primary data center (primary memory unit) to the memory unit in the secondary data center (secondary. memory unit).
1. A method in which the contents of the primary memory unit are copied to a removable storage medium such as a magnetic tape, and mailed to the secondary data center, and the contents of the storage medium are stored to the secondary memory unit.
2. A method in which data is copied from primary memory unit to secondary memory unit using a data copying function of the storage device via a communication line that connects the primary data center and the secondary data center.
In the first backup method which uses a removable storage medium, 1) the data of the primary memory unit are saved in the recording medium, 2) the storage medium is mailed to the secondary data center from the primary data center, and 3) the data in the recording medium are written to the secondary memory unit. The above troublesome work is required every time data is backed up from the primary data center to the secondary data center. The disadvantage becomes more conspicuous when a frequent back ups are required. The frequency of data back ups may be decreased to reduce the work. But there arises such a disadvantage that the data between the primary and secondary data centers often does not coincide with each other because periods during which the data of the secondary data center are old becomes long.
On the other hand, a problem of a second method using the communication line resides in how data to be communicated is appropriately selected.
Japanese Unexamined Patent Publication No. JP 2004-303122A has reported a method using a communication line. In this method, the storage device has a differential map, which is used to manage a difference of data stored in a primary memory unit and a secondary memory unit. In this method, the storage area in each of the primary and secondary memory units are divided into blocks of a certain length, and a bit is allocated to each pair of corresponding blocks in the primary memory unit and the secondary memory unit to produce a bit map. Each bit is controlled so as to indicate whether data in respective blocks of a pair to which the bit is allocated coincides with each other. When the data in respective blocks of a pair does not coincide with each other, the data copying function of the storage devices transfer data from a primary memory unit to a secondary memory unit. Thus, updated data in a primary memory unit is copied to a secondary memory unit.
A disadvantage of this method is that when the data copying function of the storage device begins to work, such as when a disaster recovery system starts its back up service, the function copies the entire data stored in the primary memory unit to the secondary memory unit through a communication line that connects the primary and the secondary data centers, requiring much time and costs for data communication. It is because when the function begins to work, the differential map indicates that the data of the entire blocks in the primary memory unit and the secondary memory unit does not coincide with each other.